Medical procedures such as amniocentesis and biopsy typically involve the insertion of a stainless steel cannula into a living body and the insertion and manipulation of certain other instruments through the lumen of the cannula.
Although the depth of penetration of the cannula into the body can be gauged by monitoring devices (such as, for example, ultra sound) external to the cannula, it would be desirable to have reference markings on the cannula itself so that the medical personnel performing the procedure can have a direct way of perceiving how deeply the cannula is being inserted.
The desired reference mark should be smooth, having minimum porosity so as to lessen the potential for entrapping microorganisms or other contaminants--the cannula being more readily autoclaved and sterilized. It would seem to be the case that markings in the nature of paint on the cannula are relatively undesirable because of relatively high porosity. Such markings would also constitute another foreign substance introduced into the body and would seem to be undesirable for that reason as well.
The desired reference mark should be precisely and economically applied, having a high degree of accuracy in placement and in thickness at a reasonable cost. It would seem to be the case that attempts to create reference marks by etching procedures, including acid etching of a groove into the cannula, are undesirable because they not only introduce a cavity where contaminants might be trapped but also because of the difficulty of precise placement at a reasonable cost.
One way to create a smooth, precise, and economical reference mark on a stainless steel tube is by using a laser. A laser can be so adjusted as to avoid penetrating, or etching a groove into, the metal tube. Instead, the laser can be set so as to discolor the surface of the metal tube by oxidizing a thin layer of the metal. A problem that makes the use of a laser difficult on thin walled metal tubing, such as a cannula, is that lasers naturally generate heat. Excessive localized heating applied to a thin walled metal tube can cause warping and burning of the metal. An unacceptable distortion of the tube interior can result.
Accordingly, it can be seen that there is a need for an apparatus by which high contrast reference marks can be placed on a cannula. Preferably, a laser, in cooperation with a cooling system and an incrementing system, would place smooth and precise marks on a cannula in such a way as to avoid the problems of excessive localized heating to the cannula walls and resultant distortion. Also, a method of using a laser system to place such marks would be required. Of course, the apparatus and method should be economical and should be able to produce precisely marked cannula to various specifications.